Radio direction finder



Feb. '7, 1939. .w. s. HINMAN JR I 2,145,375

7 RADIO DIRECTION FINDER Filed Sept. 6, 1935 Fl G. i

Snvenb or.

Fibborn ey.

Patented Feb. 7, 1939 STATES PATENT OFFICE RADIO DIRECTION FINDER States Application September 6, 1933, Serial No. 688,351

7 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalty there- My invention relates to improvements in the method and apparatus for a radio direction finder for determining the direction of a radio transmitting station, for position finding by means of cross bearings on transmitting stations,

and for use as a homing device for directing a moving vehicle to a radio transmitting station.

.[n my co-pending application, Serial Number 644,485, filed November 26, 1932, a radio direction finder for accomplishing these purposes is described. My invention herein described utilizes certain of the desirable features of the invention set forth in the application referred to, together with other desirable features whereby further desirable results are secured.

The purpose of my present invention is to simplify the direction finder, to increase its reliability, and to improve its operation, as will be evident from the following description.

The features of my invention include an adaptation of the input circuit so that it also serves as the output circuit to secure zero center indication of course with a course indicating meter, means for tuning of the loop antenna circuit to resonance to give greater sensitivity and gain in the direction finder without affecting reliability of indication.

Other objects of my invention will be set forth in the following description, it being understood that the above general statement of the objects of my invention is intended to generally explain the same without limiting it in any manner.

In the illustrated application of my invention Figure 1 shows diagrammatically one form of my apparatus having the characteristics of the invention and by which the invention may be practiced.

Figure 2 is a diagram showing two field patterns developed by a loop antenna when the phase with respect to the input of a radio receiver is reversed in rapid succession.

Several embodiments of my invention are known to me but according to that by means of which I illustrate my invention, a loop antenna I0, Figure 1, is tuned by a condenser II and connected to rectifiers l2 and I3 through blocking condensers I l and I5. The cathodes of rectifiers l2 and I3 are connected to center tapped resistor [6, zero center meter I1, and damping condenser I8. The variable center tap of I6 is connected through audio output transformer I!) to ground. Equal alternating voltages opposite in phase are applied to the plates of l2 and I3 through radio frequency chokes 20 and 2| by 5 means of an audio oscillator, consisting of a vacuum tube 23, energized from a source S, through an inductor 25 with a bypass condenser 26, coupled thereto by means of a transformer 22, one of the primaries of which is provided with 10 a tuning condenser 24, as shown, connected across the secondary of the transformer 22 is an accurately center tapped resistor 21, grounded as shown.

The center tap of loop antenna I0 is connected 15 to grid of radio frequency amplifier 28, grid leak 29 providing the ground return. The plate of 28 is connected through blocking condenser 36 to the input of radio receiver 3! and is energized from a source S through inductor 33. The out- 20 put of radio receiver 3| is connected to the output transformer I9 as shown.

The voltage developed in loop antenna I0 is applied to the radio frequency amplifier 28 from its center point to ground. The ends of the loop 25 antenna are connected to the plates of the rectifiers l2 and [3. In order that voltage be applied to 28, it is necessary that the loop be grounded at one end. The arrangement of rectifiers I2 and I3 accomplishes this end inasmuch 30 as the resistance of each rectifier becomes relatively low when a positive voltage is applied to the anode thereof while the resistance of the other of the rectifiers becomes relatively high when a negative voltage is applied to the anode 35 thereof. Thus, as the voltages applied to these plates are equal but opposite in phase, one rectifier has low resistance when the other rectifier has high resistance, and vice versa. For any specific alternation of the switching voltage sup- 4o plied by the oscillator tube 23 and its associated apparatus, the loop antenna is grounded through the particular rectifier having low resistance, and this ground is reversed once each half cycle of the switching frequencies, the rectifiers being 2.1- 45 ternately of high and low resistance. Thus the phase of the loop antenna current is reversed with respect to the radio frequency amplifier each half cycle of the audio switching voltage.

When the system is connected in the manner 50 shown, two field patterns are produced as shown respectively by 34 and 35 in Figure 2. The modified form of figure-of-eight is obtained by the unsymmetrical ground connection, the vertical effect of the loop antenna being thereby superposed upon its normal figure-of-eight characteristic.

Having described the apparatus, its operation as a direction finder may now be considered. When no radio wave develops voltage in the loop antenna rectifiers I2 and I3 pass current alternately due to the switching voltage from audiopressed on the loop antenna, this signal is amp1ified by the radio receiver. The amplitude of this signal at the output of the radio receiver is proportional to the field pattern and the-orientation of the loop antenna at a given instant. When rectifier I2 passes current, a certain field pattern is developed as shown in Figure 2 and clearly explained in my copending application Serial Number 644,485, and the current through [2 and through course indicator I I is proportional to the output voltage due to that field pattern as applied from output transformer I9 plus the voltage from the audio oscillator. Similarly when rectifier l3 passes current, another field pattern is developed and the current through I3 and course indicator I1 is proportional to the output voltage due to the other field pattern plus the audio oscillator, and these last currents fiow through I! in a reversed direction. Since the currents, due to the audio oscillator switching voltage, are equal and flow through I! in opposite directions, they cancel and the net currents deflecting the course indicator. I! right and left are proportional to the difference between the respective output voltages due to the two loop antenna field patterns. It will be seen from Figure 2 that voltage from the two field patterns are equal for two orientations of the loop antenna (see 36 and 31) the currents through I! At the other being then equal and opposite. points, such as 38, 39, 40, and 4|, the two field patterns vary considerably, and the course indicator I1 deflects to the right in proportion to one field pattern and to the left in proportion to the other field pattern giving right or left indication.

As distinguished from the construction and operation of the devices described and illustrated in my copending application Serial Number 644,485, the rectifier tubes I2 and I3 have a dual function in the present invention, not only serving in combination with the audio switching voltage to reverse the field pattern of the loop antenna, but also serving to rectify and at the same time reverse synchronously the output current of the receiving set through the course indicating instrument. Considerable equipment required in the former invention is thereby eliminated and at the same time, in view of the reduction of apparatus, the reliability of the direction finder is increased.

Furthermore, an advantage in this arrangement over that disclosed in my copending application is that in this arrangement the loop antenna is tuned to resonance without sacrificing reliability, the phasing of currents being made practicable by inserted or inherent resistance in the loop antenna circuit making the vertical effect of the nism is so connected to a loop antenna and to a radio'receiver that the phaseof the voltages de- Veloped in the loop antenna is rapidly reversed with respect to the radio receiver. The relative intensities of these two sets of voltage depends upon the orientation of the loop antenna. When the plane of the loop antenna is perpendicular to the direction of the station received, the two sets of voltages are equal. If the loop antenna is rotated, say, to the right, one set of voltages becomes greater than the other, while if the loop antenna is rotated to the left, the reverse is true.

The receivingset amplifies and detects these voltages and applies the resultant audio frequencies to a zero center course indicating meter in combination with the input switching arrangement.

A source of alternating power is provided to reverse the phase of the loop antenna current with respect to the radio receiver and to operate thezero center course indicator in such a way that .for one connection of the loop'antenna to the receiving set, current passes throughthe course indicator in one directionand for the second connection of the loop antenna, the flow of current through the course indicator is reversed. When a signal is being received, the output voltage of the radio receiver is also applied to the input circuit arrangement and to the'course indicating meter, and since this voltage is at all times proportional to the orientation of the loop antenna for each of the two connections in the input circuit, the course indicator is deflected right or left in proportion to the difference between the voltages developed in the output circuit for each of the two connections in the input circuit.

The method of using this direction finder is now evident. When the plane of the loop antenna is perpendicular to the direction of the transmitting station being received, the two sets of voltages alternately applied by the loop antenna to the radio receiver are equal. Then the currents applied through the course indicator from the output of the receiver set are also equal. Since these two sets of currents fiow through the course indicator in opposite directions, the resultant current through the course indicator is zero. The latter may, therefore, be of the zero center type and will read on course for the condition outlined. If the loop antenna is now rotated to the left, the current passing through the course indicator in one direction will be greater than in the reverse direction. The course indicator may therefore be connected so that its pointer will deflect to the left. Now if the loop antenna is rotated to the right, the course indicator will obviously deflect to the right.

If, however, the loop antenna is rotated 180, the pointer will also read on course, but when the loop antenna is rotated right or left, the direction of the pointer movement is reversed. This direction finder therefore gives not only an indication of the direction of the received signal, but also sense of deviation of the line of flight of the airplane from this direction.

By providing means whereby one may rotate the loop antenna, this direction finder may be used not only as a homing device but also for triangulation by getting bearings on two or more ground stations.

While I have described certain embodiments of my invention, it is neither to be restricted to the particular embodiments described nor be limited by any description thereof except as defined by the scope of the claims appended hereto.

I claim as my invention:

1. In a difierential current indicator, the combination of a center tap coil, an amplifier-rectifier unit the input of which is coupled to the center tap of said coil, a pair of rectifiers the anodes of which are coupled to the end terminals of said coil and the cathodes of which are coupled to the output of said amplifier-rectifier unit through a balanced resistor, means for energizing the space current paths of said rectifiers and balanced resistor with alternating current of opposite phase, and means in shunt with said balanced resistor responsive to the differential current passing through said balanced resistor from said rectifiers and from the output of said amplifierrectifier unit.

2. In a directional radio receiving system, the combination of a center tapped loop antenna, a radio amplifier the input of which is coupled to the center tap of said loop, a pair of rectifiers, the anodes of which are coupled respectively to the end terminals of said loop, means for energizing the space current paths of said rectifiers with alternating current of opposite phase, a meter and a center tapped resistance parallel connected between the cathodes of said rectifiers and means for applying to the center tap of said resistance potentials developed by the output of said amplifier.

3. In a radio direction finder, a single tuned coil antenna with two end terminals and a center tap terminal, a radio receiving set with input and output circuits each having two terminals, one terminal of said input circuit being connected to said coil antenna center tap and the other terminal being connected to ground, a resistor having a grounded center tap connected between the cathodes of two rectifying and grounding means for alternately grounding said coil antenna end terminals through a separate half of said resistor, a zero center current indicating means connected across said resistor, said output circuit of said radio receiving set being alternately closed by said rectifying and grounding means such that the current fiowing in said output circuit passes through said current indicating means alternately in one or the other direction in synchronism with the alternate grounding of said coil antenna end terminals, whereby the indications of said zero center indicating means represent the angular position of said coil antenna about a vertical axis.

4. In a radio direction finder, a single tuned coil antenna, amplifying and detecting means, a pair of rectifying means for alternately impressing the voltage induced in opposite halves of said coil antenna upon the input of said amplifying and detecting means, a zero center current indicating means, connected between the cathodes of said rectifying means and the output of said amplifying and detecting means whereby the rectified output current is passed through said current indicating means alternately in opposite directions such that the current in each direction is proportional to the voltage induced in the corresponding half of said coil antenna, whereby the indications of said zero center indicating means represent the angular position of said coil antenna about a vertical axis.

5. In a radio direction finder the combination of a radio receiver, a loop antenna having center tap and end terminals, the center tap being connected to the input of said receiver, a pair of rectifiers the anodes of which are coupled to the end terminals of said loop respectively and the cathodes of which are connected together by a center tapped resistance the center tap of which is connected to ground and coupled to the output of said radio receiver, a meter connected between the cathodes of said rectifiers and means for producing across the anode-cathode paths of said rectifiers alternating voltages, the phase of the voltage applied to each rectifier being 180 out of phase with respect to the voltage applied to the other rectifier whereby the space pattern of said loop is reversed periodically as a function of the alternating voltages applied to said rectifiers and whereby the deflection of said meter indicates the directional and angular location of the source of radio energy received by said loop with respect thereto.

6. In a directional radio receiving system, the combination of a center tapped loop antenna, a radio receiver the input of which is coupled to the center tap of said loop, unidirectional space current devices for automatically alternately grounding the end terminals of said loop antenna, and means connected between the cathodes of said space current devices and coupled to the output of said receiver responsive to the difierential output of said receiver and the currents passing through said unidirectional space current devices.

'7. In a directional radio receiving system, the combination of a center tapped loop antenna, a radio receiver the input of which is coupled to the center tap of said loop, a pair of rectifiers, the anodes of which are coupled respectively to the end terminals of said loop, means for energizing the space current paths of said rectifiers with alternating current of opposite phase, and means connected between the cathodes of said rectifiers and coupled to the output of said receiver responsive to the differential output of said receiver and the currents passing through said rectifiers.

WILBUR S. I-IINMAN, JR. 

